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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

466

GLYCERINE

of constant weight. From i to 1-5 gram of the glycerine is then dropped
on to the asbestos so that it is uniformly distributed ; after being weighed
again, the glass is left in the desiccator at the above pressure until the
weight is constant (usually about 48 hours at 15° are required). The loss
of weight represents water.

9. Glycerine.—Either of two methods may be used1:
A. ACETYLATION METHOD, applicable to crude glycerine, provided it
contains not more than 50% of water.
Reagents required:

(a)  Acetic anhydride (puriss.), which should be carefully examined as
to purity and which in a blank esterification experiment should not require
more than 0-1-0-2 c.c. of normal soda, and which should turn only slightly
brown when boiled for an hour with sodium acetate.

(b)  Pure dry sodium acetate, obtained by fusing the salt in a platinum
dish, powdering it rapidly and storing in a closed vessel in a desiccator.
It should be absolutely free from moisture.

(c)  Normal caustic soda solution, which should be very carefully pre-
pared with well-boiled water and should be quite free from, carbonate.

(d)  Normal sulphuric acid.

(e)  Phenolphthalein solution, containing 0-5 part in 100 parts of alcohol
and neutralised.

Procedure. In a round-bottomed flask of about 120 c.c. capacity, well
washed and dried, 1-25-1-50 gram of the glycerine is rapidly weighed, 3
grams of the sodium acetate and 7-5 c.c. of acetic anhydride being added.
The flask is then connected with a small reflux condenser by means of a
ground joint or a rubber stopper (the latter should be first purified by
exposure to the vapour of boiling acetic anhydride) and the liquid boiled
gently for about an hour. It is then cooled somewhat and 50 c.c. of recently
boiled hot water (at about 80°) added by way of the condenser tube, the
liquid being shaken and heated, if necessary—but not above 80°—until
solution is complete (excepting for a few black flocks due to impurities).
The condenser tube is washed down with a little boiled water, the flask
detached and the stopper or ground joint also washed down. The liquid is
filtered into a flask holding about a litre, the original flask and filter being
thoroughly washed with boiled, cold water. 2 c.c. of phenolphthalein
solution are next added and the liquid neutralised with the normal caustic
soda solution (to a faint yellowish-red coloration), care being taken to
shake the flask continually while the alkaline solution is run in from a
burette.

After the neutral point is reached, a further quantity of 50 c.c. of normal
caustic soda is added, the flask being then closed with a stopper traversed
by a long glass tube to act as reflux condenser, and the liquid boiled gently
for 15 minutes, then cooled rapidly and the excess of alkali titrated with
the normal acid until the original yellowish-red tint reappears.            • j

At the same time a check experiment is made under the same conditions.

From the quantity of caustic soda used to saponify the triacetin (differ-
ence between the volume of caustic soda added after neutralisation and the
1 Recommended by the Committee mentioned above,eviously well washed with acid and